RE: How to stop planned production order from generating a purchase order ?
Looks like my previous response did not post.
As a work around you could change the order type for such an item to production but then you would always have to convert it to PO manually and no vendor will be selected and the lead time will not be known so you would then have to add some safety lead time -so not the best option to my mind, unless for a specific item/situation.
Negative days is the lateness tolerance you are willing to live with, so mrp will not trigger a new order in that window but will suggest expediting messages. However is that always going to be the same for all orders for that item?
Positive days will not help in this situation.
In the situation you describe it is best to turn on the massages. Mrp does not like things out of balance but it believes what you tel lit.
So if you say the lead time is 10 days but you also say you need it tomorrow it is not happy but it will action the demand, but it will also believe you intend to do something to resolve the conflict and will thus send you messages .e.g to cancel the order it just planned or to expedite the order already planned.
So turn on messages and get used to reviewing those first. But expect some performance impact on mrp run -probably OK if you do it overnight out of hours.